Many musical instruments such as electric guitars are used in conjunction with electronic amplifiers and other electronic devices to render an audible signal that has a desired sound quality and loudness. Typically, an electric guitar will be used with a guitar amplifier comprising a preamplifier, a power amplifier and one or more speakers, all built into a common cabinet or in a set of cabinets designed to be stacked or used together. Guitar amplifiers are often designed to produce very loud sound output from the speakers. Guitar amplifiers designed for live performances may be rated at power levels of 50 to 100 Watts or even considerably more.
However, a large proportion of guitar amplifiers are used by novice guitar players who desire to use the amplifier for practice purposes and who will often play relatively quietly along with recorded music, training media, etc. Novice players frequently play softly to avoid disturbing other people. As much as possible, it is desirable that the sound level of the guitar playing be controlled to low levels while still maintaining the sound characteristics, such as distortion and tone characteristics, that the combination of the guitar, amplifier and speaker would exhibit at higher volumes. Even very experienced guitar players will often play at low to medium volume levels when not engaged in a live performance, such as when practicing or recording. In yet other instances, a guitar player using an electric guitar may be playing alongside other instruments that are not electronically amplified, such as unamplified acoustic guitars.
With a typical guitar amplifier, it is difficult to control volume at relatively low levels. For example, in the usual case where a volume control knob on a guitar amplifier is labeled to be adjustable from “0” (no output) to “10” (maximum output), adjusting the volume output to a comfortable level may require carefully setting the knob position to somewhere between “0” and “1”. Often the adjustment is extremely erratic, meaning that minute adjustments in the position of the knob may cause the volume to jump stepwise rather than smoothly. The volume control knob on a guitar amplifier typically controls a potentiometer in the electronic circuitry of the amplifier. If the potentiometer is dirty or noisy and the contact resistance between the wiper and the resistance element varies as the potentiometer is adjusted, then it is even possible that turning the knob towards a lower setting may cause the volume to actually increase in an erratic fashion. This effect is more pronounced as the knob position approaches the “0” position. With some volume controls, it may not be possible to achieve an arbitrarily chosen desired low volume level.
Even where the volume control potentiometer is ‘clean’ and volume varies smoothly with movement of the knob coupled to the potentiometer, the range of desirable volume settings are compressed to less than 10% (and often less than 2%) of the available motion of the volume control knob. This may require the user to use extreme finesse in positioning the knob, which makes use of the amplifier more difficult and frustrating. All of these problems persist even where potentiometers are used which employ a logarithmic taper.
Some players naturally attempt to alleviate these problems by simply setting the volume knob on the amplifier to some approximate level and then exercising fine control over the sound volume by using one or more volume control potentiometers on the guitar itself. Unfortunately, this approach has at least two undesirable side effects. Often, distortion or other effects that are to be applied to the guitar's electrical output signal are very dependent upon the signal level coming from the guitar. Reducing the output level from the guitar by using the volume control potentiometer of guitar can dramatically alter many characteristics, other than just volume, of the sound that ultimately comes out of the speaker. For example, turning the guitar output down can dramatically reduce or eliminate the distortion or overdrive that is desired for some types of music. Thus, the user is deprived of being able to hear, at low volumes, the same sound signal that the amplifier would produce at higher volumes. Another problem with using the volume control potentiometer of a guitar to adjust the overall volume of the guitar amplifier combination is that this can alter the frequency response of the guitar, typically causing the high frequencies to be selectively attenuated as the guitar volume is turned down. This is largely due to the inductive characteristics of the guitar pickups, which are essentially wire coils around ferromagnetic cores positioned near the guitar strings to act as transducers. As the volume knob is turned down, the guitar's electrical output signal effectively loses treble content or higher frequency content, resulting in a disappointingly dull sound.
Another solution to this problem that has been proposed involves attenuating the electrical signal between the power amplifier and a speaker, essentially dissipating a large proportion of the amplifier's power output into a resistive dummy load. While this may enable the volume control to be operated in its midrange, most of the large amount of signal power out of the power amplifier is wasted in the form of heat. Devices that operate in this manner are quite expensive, wasteful of electrical power, and, if not designed properly or subject to failure, can damage power amplifier output stages. Components in the output stages are subjected to greater electrical, mechanical or thermal stress and can wear out or fail much faster.
Thus, there is frequent and widespread need to be able to control the volume of instrument amplifiers to enable playing softly or at moderate volumes, even though the amplifier may be capable of producing sounds in excess of 100 decibels.